A system has been devised in recent years in which a plurality of sensors are installed in structures such as buildings and bridges, and a center server collects obtained measurement values to observe displacements generated in the structures (structural health monitoring). In this case, each sensor transmits the obtained measurement value to the center server. Thus, a wired or wireless ad-hoc network may be applied to such a sensor network.
Clock time data may be used for measuring some physical quantities that are measured in the sensor network. For example, in the case where an acceleration sensor is used for measuring a magnitude of deflection of a structure or a shock of an earthquake, clock time information is used for calculating an acceleration. An allowable difference among clock times of sensors included in the same sensor network varies depending on a sampling interval and a magnitude of an allowable error. For example, if the sampling interval is 5 ms and the allowable error is 5%, differences in clock times among the sensors are maintained within 250 μs or less. The differences in clock times however occur among the sensors even after the sensors are synchronized in clock time, due to deviation of a frequency on which the sensor operates, or the like. For this reason, clock time synchronization is periodically performed among the sensors included in the same sensor network. Methods for synchronizing clock times among a plurality of sensors include a method in which individual sensors adjust respective clock times by using a radio clock, a GPS (Global Positioning System), an NTP (Network Time Protocol), or the like. In addition, an RBS (Reference Broadcast Synchronization), a TPSN (Timing-sync Protocol for Sensor Networks), an FTSP (Flooding Time Synchronization Protocol), and the like are known as methods accompanied by a control in a Media Access Control (MAC) layer.
As a related art, a method is known in which a first device transmits a token containing clock time information to a second device, and the second device corrects a clock time thereof using the token and transmits a token containing clock time information to which a processing time period is added, to a third device. Furthermore, a transmitting device is also known that transmits, upon receiving a clock time data transmission request from a receiving device, a fixed-length frame containing clock time data converted into a code that will not be subjected to zero insertion in binary coding and a transmission delay time, to the receiving device.
As an art relating to an ad-hoc network, a node device has been proposed that stores identification information of a frame to be transmitted while associating it with a neighboring node device being a transmission destination, and with a transfer source that first transfers the frame to be transmitted. This node device compares, upon receiving a frame, an identification number of the received frame with an identification number of a frame to be transmitted to check a transmission route of the frame for a loop.
For example, some literatures are known such as Japanese Patent Laid-Open No. 9-83608, Japanese Patent Laid-Open No. 2006-93967, and International Publication No. WO 2011/013165.
In the case where individual node devices correct respective clock times using the radio clock or the NTP to adjust the clock times among the node devices, magnitudes of errors that occur among the node devices are greater than an error that is allowed in a sensor system used for measuring a physical quantity. Therefore, even when the individual node devices adjust the clock times using the radio clock or the NTP, the clock time synchronization is not performed to a precision required in the sensor system. In the case where individual node devices synchronize the clock times using the GPS, although the synchronization among the node devices is performed with high precision, the individual node devices each consume more power, which is not suitable for node devices used in the sensor network. Furthermore, hardware used in the method for synchronizing clock times such as the RBS, the TPSN, and the FTSP, may be customized because the methods each need an unique process to be performed. A problem of the customized hardware is in that there is no compatibility among chips included in node devices. Furthermore, the methods for synchronizing clock times such as the RBS, the TPSN, and the FTSP are difficult to apply to node devices used in the sensor network because there are problems with them such as increased power consumption in the clock time synchronization process, and a small number of node devices that can synchronize the clock times.
The method for transmitting and receiving a fixed-length frame containing the clock time information requires a frame to be transmitted and received to have a fixed length, which may be difficult to be applied to the sensor network. Furthermore, the method in which one of the devices in the network adjusts the clock time using a token is not applied to the case where a large number of node devices are included in the ad-hoc network.